test: add weather logic test suite

tests/forecast-utils.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+
+import { getEffectiveHourlyWeatherCode, getForecastConditionKey, isForecastNight } from "@/lib/forecast-utils";
+
+describe("forecast utilities", () => {
+  it("promotes dry weather code to rain when precipitation probability is high", () => {
+    expect(
+      getEffectiveHourlyWeatherCode({
+        weatherCode: 0,
+        precipitation: 0,
+        precipitationProbability: 100,
+      }),
+    ).toBe(61);
+  });
+
+  it("keeps clear weather code when there is no precipitation signal", () => {
+    expect(
+      getEffectiveHourlyWeatherCode({
+        weatherCode: 0,
+        precipitation: 0,
+        precipitationProbability: 10,
+      }),
+    ).toBe(0);
+  });
+
+  it("recognizes night hours from sunrise and sunset", () => {
+    const day = {
+      sunrise: "2026-06-15T05:00",
+      sunset: "2026-06-15T21:00",
+    };
+
+    expect(isForecastNight("2026-06-15T03:00", day)).toBe(true);
+    expect(isForecastNight("2026-06-15T12:00", day)).toBe(false);
+    expect(isForecastNight("2026-06-15T22:00", day)).toBe(true);
+  });
+
+  it("maps storm and rain weather codes to condition keys", () => {
+    expect(getForecastConditionKey(61)).toBe("forecast.condition.rain");
+    expect(getForecastConditionKey(95)).toBe("forecast.condition.thunderstorm");
+  });
+});